Bead-Pull RF Measurement System RF Measurement System Jackline Koech Electrical and Computer Engineering Department University of Massachusetts Amherst, Massachusetts, 01003

Bead-Pull RF Measurement System

Jackline Koech

Electrical and Computer Engineering Department

University of Massachusetts

Amherst, Massachusetts, 01003

Advisor: David Peterson

Antiproton Source Department, Accelerator Division

Fermi National Accelerator Laboratory

Batavia, Illinois, 60510

Summer Internships in Science and Technology(SIST)

Internship Program, 2011

August 11, 2011

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Abstract

Bead-Pull is a commonly used Radio Frequency (RF) field measure-

ment technique. RF field measurements play an important role in qual-

ifying any RF cavity. They are used in evaluating the field distribution

inside a resonant structure and in tuning them to obtain the required

field flatness [1]. The Bead-Pull system consists of a small dielectric

or metallic bead being pulled through a cavity while the electric field

measurement in the cavities is done. A step motor and a pulley system

guide the motion of the bead through the cavity while a Network An-

alyzer is used to take the RF measurements. We wrote a program in

National Instruments’ Laboratory Virtual Instrumentation Engineering

Workbench (LabVIEW) to control the hardware of a Bead-Pull system.

The software will coordinate the step motor’s movement, acquire data

via the Network Analyzer and process the data as required. This paper

describes the development and the testing of this software.

Key Words: Bead-Pull RF Measurement System, Step Motor, LabVIEW,

Network Analyzer.

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Contents

1 Introduction 4

1.1 Fermilab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Bead-Pull RF System . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Tools and Methods 5

2.1 Network Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 S-Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Step Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 LabVIEW Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 Network Analyzer VI . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.2 Step Motor VI . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.3 Overall Bead-Pull System VI . . . . . . . . . . . . . . . . . . 9

3 Data and Conclusion 10

4 Acknowledgments 12

List of Figures

1 Applied Motion Instruments . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Front Panel of the Bead-Pull Measurement System . . . . . . . . . . . . 10

3 Resonance Frequency Measurement . . . . . . . . . . . . . . . . . . . . 11

4 Pill Box Test Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Graph of Resonance Frequency Measurements of the Pill Box Test Cavity 13

List of Tables

1 Network Analyzer Commands . . . . . . . . . . . . . . . . . . . . . . 8

2 Step Motor Commands . . . . . . . . . . . . . . . . . . . . . . . . . . 9

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1 Introduction

1.1 Fermilab

Fermi National Accelerator Laboratory (Fermilab) enhances the understanding of

the nature of matter and energy by providing the resources for researchers to conduct

research in high energy Physics and other related disciplines. I interned in the

Antiproton Source (Pbar) department which is part of the Accelerator division.

While here, I developed a software to be used in controlling the Bead-Pull Radio

Frequency (RF) measurement system which will be used in the development and

testing of cavities in the Linear Accelerator (LINAC) department of the Accelerator

division.

1.2 Bead-Pull RF System

Bead-Pull Radio Frequency (RF) measurement system consists of a small dielectric

or metallic bead being pulled through a cavity while electric field measurements in

the cavity are taken. RF field measurements play an important role in qualifying

any RF cavity. These field measurements are also used in the tuning of the cavities

to obtain the required field flatness. The Bead-Pull method is based on the classical

Slater perturbation theory which states that if any resonant cavity is perturbed by a

small bead, its resonant frequency shifts from the original frequency. This frequency

shift is proportional to the combination of the squared amplitudes of the electrical

and magnetic fields at the location of the bead [1]. This relationship is given by

equation 1 [2].

ω2 − ω02

ω02

= k∫

∆r

µH2 − εE2

2Udv (1)

where ω and ω0 are the new and the original resonant frequencies respectively, k

is a constant determined by the shape of the bead, µ is the permeability constant,

ε is the permittivity constant, U is the energy stored in the cavity while E and

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H are the electric and magnetic field amplitudes respectively. From this equation,

we realize that, if the magnetic field is zero (which is the case along the center

of the cavity), the electric field is directly proportional to the change in resonant

frequency. Therefore, if the change in resonant frequency is known, the electric field

can be determined by moving the bead along a line in the cavity.

2 Tools and Methods

The hardware of Bead-Pull system consists of a Network Analyzer, a Step Motor,

and miscellaneous parts such as the pulleys and the thread. The Step Motor moves

a small metal or a dielectric bead linearly through a cavity. The Network Analyzer

measures the change of resonant frequency which is related to the local electrical field

where the bead passes. This information is useful to evaluate the field distribution

or tuning of an RF resonant structure. A program is needed to coordinate the Step

Motor’s movement,the data acquisition of the Network Analyzer and to process the

acquired data.

2.1 Network Analyzer

A Network Analyzer is an instrument that measures network parameters of electrical

networks. It commonly measures the scattering parameters (S-parameters) and is

mostly used at high frequencies. This is because, at high frequencies, the wavelength

of signals of interest is comparable to or much smaller than the length of conduc-

tors [3] and is therefore harder to measure voltage and current. Network analyzers

can measure both linear and non-linear behavior of devices. Network Analysis is

concerned with the accurate measurement of the ratios of the reflected signal to the

incident signal as well as the transmitted signal to the incident signal.

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2.1.1 S-Parameters

S-parameters are used to characterize high frequency circuits in place of the impedance

or admittance parameters that describe the low frequency circuits. S-parameters re-

late the transmitted or reflected waves to the incident waves while Z-parameters

(impedance parameters), for example, relate port voltages and currents. An N-port

device has N2 S-parameters. Parameters such as S11 and S22 are referred to as the

reflection coefficients because it is the ratio of the wave coming out of a given port to

the wave incident at that port. The other S-parameters such as S12, S21 are referred

to as the transmission parameters because they refer to the ratio of the wave coming

out of a different port than was incident into.

2.2 Step Motor

(a) HT23-601 Step Motor (b) Step Motor Driver (c) Power Supply

Figure 1: Applied Motion Instruments

We used a Step Motor to move the bead across the cavity. A Step Motor consists of

a permanent magnet rotating shaft called a rotor. The Step Motor converts digital

pulses into mechanical shaft rotation. Every rotation is divided into many discrete

steps and it can stop at any step making it suitable for small movements. It has

an excellent response to starting, stopping and reversing. However, it is hard to

operate at high speeds and resonance can occur if not properly controlled. We used

the Applied Motions’ HT23-601 Step Motor shown in Figure 1a which is suitable for

a wide range of motion control applications. It was set to 20,000 steps per rotation

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and we observed that for our pulley system setup, approximately 255 steps were

required to move the bead by 1mm using this Step Motor. The Step Motor was run

using the Applied Motions ST5-Plus Step Motor Driver shown in Figure 1b. We

also used Applied Motions 24V PS150A24 Power Supply (shown in Figure 1c) to

power the Step Motor and the Step Motor Driver.

2.3 LabVIEW Program

The entire program was written in National Instruments Laboratory Virtual Instru-

mentation Engineering Workbench (LabVIEW) software, a graphical programming

environment that contains comprehensive set of tools for acquiring, displaying, an-

alyzing and storing data. A LabVIEW program is called a virtual instrument (VI)

and each VI has both a front panel and a block diagram. A front panel displays the

controls and indicators while a block diagram shows how all the controls and the

indicators are connected together to achieve the desired purpose. In order to develop

the Bead-Pull software, we started with learning the commands necessary for per-

forming the required operations for both the Step Motor and the Network Analyzer

by reading their programming manuals and then integrating the commands to form

VIs that control each individually. We then finally combined all the VIs to form an

overall LabVIEW measurement program that moves the bead via the Step Motor

and takes the measurements via the Network Analyzer.

2.3.1 Network Analyzer VI

The Network Analyzer VI provides an option of selecting the Network Analyzer

model that is being used to take the measurements, does a single sweep and takes

specified measurements (error corrected data or resonant frequency). For the reso-

nant frequency measurements it takes the data at a given point a number of times

specified by the user and averages them for better measurements. This VI also dis-

plays the measurements on the screen and converts them into the desired format.

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Table 1: Network Analyzer Commands

Command HP 4396A Agilent8720ES

HP 8751

Single Sweep SING SING SINGIn hold mode? HOLD? HOLD? HOLD?Output error corrected data OUTPDATA? OUTPDATA OUTPDATA?Wait for clean sweep NE1 WAIT NE1

Number of data points/sweep POIN? POIN? POIN?Returns the start frequency STAR? STAR? STAR?Returns the stop frequency STOP? STOP? STOP?Outputs active marker values OUTPMKR? OUTPMARK OUTPMARK?Previous operations complete? NE1 OPC? NE1

1 No Equivalent

The program can be used with any of these three Network Analyzers: Agilent 8720,

HP 4396A and HP 8751. Table 1 outlines the similarities and differences between

the commands for the three Network Analyzers. To perform a single sweep, we

issue the command SING which is similar for all the analyzer models. The program

needs to wait until the Network Analyzer is done sweeping and is in the hold mode

before the data is read from the Network Analyzer. To ensure this, the program

issues another command, HOLD? which returns 1 when the analyzer is in the hold

mode. Once the response is 1, the program starts taking measurements by issuing

the output data command (to get the error corrected data) or the output marker

value command to get the resonant frequency.

2.3.2 Step Motor VI

The Step Motor VI controls the motion of the bead via the rotation of the Step

Motor. This VI calculates the number of steps required for one step size and moves

the motor by that number of steps using the Feed to Length (FL) command. Table

2 shows a list of the commands used in this VI.

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Table 2: Step Motor Commands

Command Function Explanation

AC Acceleration Rate Sets the Acceleration Rate. AC5 will set theacceleration rate to 5 Rev/s2

AR Clear Alarm Clears Alarms and drive faults and leaves themotor in the disabled state.

CC Change Current Also known as the running current.DE Deceleration Rate DE5 sets the Deceleration Rate to 5 Rev/s2

FL Feed to Length FL20,000 will move the motor 20,000 steps inthe clockwise direction

ME Motor Enable Restores drive current to motor.PM Power up Mode It sets or requests the power up mode of the

drive.RE Restart or Reset This restarts the drive by resetting default con-

ditions and reinitializing the drive with thestartup parameters. It leaves the drive in adisabled state.

SM Stop Move Stops motion, but does not stop any WAITcommands

ST Stop Stops the motion immediately using the decel-eration rate set by the maximum acceleration

VE Velocity VE5 would set the velocity to 5 Rev/s

2.3.3 Overall Bead-Pull System VI

The overall Bead-Pull software sets the parameters of the Step Motor, such as

the velocity, the acceleration rate and the deceleration rate. It begins taking the

measurements starting with the original position of the bead. This VI then calls

the Step Motor VI to step the bead by the amount specified by the user in the step

size input box and calls the Network Analyzer VI to take the measurements. This

procedure is repeated while the data collected is appended to the previous data until

the bead reaches the stop distance set by the user. Once it reaches the stop position,

it writes all the data collected into a spreadsheet file whose location and name has to

be specified by the user. It also displays a graph of the resonance frequencies against

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the data points on the front panel if it is in the resonant frequency measurement

mode. Finally, it moves the bead back to the start position. Figure 2 shows the

front panel of this VI.

3 Data and Conclusion

Figure 2: Front Panel of the Bead-Pull Measurement System

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Figure 3: Resonance Frequency Measurement

We tested the program using a pill box test cavity (shown in Figure 4) which has

a radius of 12.057cm and a length of 1.575cm with beam pipes attached on both

sides. First, we were able to confirm that our software moved the bead as expected

and that the steps could be repeated accurately. By taking the measurements while

watching the measurements displayed on the Network Analyzer, we were also able

to confirm that the program read the measurements correctly from the Network

Analyzer. Figure 5 shows the graph of the resonant frequency measurements that

we acquired using this test cavity. To get the resonant frequency at each datapoint,

we measured the S21 parameter as shown in Figure 3. with the resonant frequency

being the frequency at which the peak of the S21 occurs. As anticipated, when the

bead enters the cavity the resonance frequency shifts from the original frequency

(measured when the bead is outside the cavity) and reaches maximum at center of

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the cavity as shown in Figure 5. However, the noise level was very high in our first

set of data. This was contributed to by the vibration of the thread, attached to the

bead, as the bead was moved by the Step Motor. We reduced these vibrations by

tightening the thread. We also included an option in the program that allows the

user to input the number of times to the average data at each datapoint for more

consistency. We ran the program with different values for averaging and we found

that three was the optimum value that improved the data to a great extent without

slowing down the program too much.

Figure 4: Pill Box Test Cavity

In conclusion, we have successfully developed a software that controls the Bead-

Pull RF Measurement System by moving the bead and taking the measurements

accurately.

4 Acknowledgments

I would like to thank Fermilab and the SIST committee for the opportunity to

participate in the SIST program again this summer. Many thanks to my supervisor,

David Peterson for the great mentorship throughout the summer and Ding Sun and

his group for setting up the project and making sure that it was working. I would

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Figure 5: Graph of Resonance Frequency Measurements of the Pill Box Test Cavity

also like to extend my gratitude to my mentors, Cosmore Sylvester and Mayling

Wong for making sure everything was running smoothly and Dr. James Davenport

for the important information on paper writing and presentation. Finally, I would

like to thank all the employees at the Antiproton Source Department for all their

support and all those who made my experience at Fermilab a memorable one.

References

[1] S.V. Kokil D.S. Rajpoot S.K. Chauhan, S. Raghavendra and S.C. Joshi. Devel-

opment of Automated Test Bench for Measurement of the Field Distribution in

Single Cell Elliptical Superconducting Cavity. Raja Ramana Center for Advanced

Technology, Indore (MP), INDIA.

[2] Edward Ginzton. Microwave Measurements. 1957. p 448.

[3] Agilent Technologies. Agilent network analyzer basics. Website, August, 2004.

http://cp.literature.agilent.com/litweb/pdf/5965-7917E.pdf.

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